Just after the button is pushed to flush the toilet a strange noise can be heard. The toilet cistern is boxed into a cabinet set up. I can access the cistern quite easily and have had a look inside. The noise sounds like something getting stuck or parts rubbing together but I cant see anything obvious that would be causing this. The toilet flushes as normal.

There is a normal looking plastic ball valve in the cistern and another largeish plastic part that has a plastic tube comming out the top which is connected to the button to flush the toilet -???

Any suggestions as to what is wrong and how I fix it would be much appreciated.